Bidirectional semiconductor switching devices of the gate-controlled type of controlling current are well known. In particular, one type of silicon-controlled rectifier comprises a silicon semiconductor body of five zones of alternating conductivity type from one major surface to the other. The gate or control contact is made to a zone adjacent to a terminal zone. Conduction in either direction is enabled, depending on the respective polarity of the voltage at the terminal zones when a trigger voltage is applied to the gate zone.
Ordinarily, the bidirectional switching device just described is triggered by separately generated gate voltages applied from external sources. For certain applications, it is desirable that there be provided a two-terminal device responsive to voltages applied across the device itself which are within a relatively narrow range to switch the element to a low impedance or ON condition. It is desirable that such a device be responsive to voltages of either polarity.